A great variety of modern electronic devices use continually rotating electrical motors. Accordingly, a wide variety of types of motors and innovations applicable to motors have been developed to meet the needs of the diversity of demand. As the surrounding technology has advanced, the demand for brushless DC motors of greater efficiency and of smaller size has increased. Particularly, high efficiency, as gauged by a high torque output to coil winding resistance ratio, is considered to be critical. Of course, a motor of higher efficiency uses less power to produce a given amount of torque than does a comparable motor and is thus especially desirable.
The Brushless DC Motor Assembly with Improved Stator Pole of U.S. Pat. No. 4,499,407, issued to Macleod, is an example of a prior art brushless DC motor which, while offering several advantages over other prior art motors and being quite useful for its intended purpose, is not small enough in the height dimension or efficient enough, and does not operate smoothly enough to meet some of the specific needs which have developed in the industry since the issuance of the patent.
Several of the recent innovations relating to brushless DC motors have been improvements regarding changes in the number and/or configuration of permanent magnetic poles and electromagnetic poles utilized in the design. U.S. Pat. No. 4,774,428, issued to Konecny, teaches a motor having 3(2n+1).+-.1 permanent magnet poles and 3(2n+1) electromagnetic poles (and, consequently, 3(2n+1) "slots" between electromagnetic poles). The Konecny patent offers a motor of improved size and efficiency. However, still greater improvements in size and efficiency are desirable.
Furthermore, the solution to the efficiency problem provided by the Konecny patent results in a motor with an asymmetrical coil winding pattern and, therefore, significant asymmetrical radial forces. These asymmetrical radial forces are particularly offensive in applications where the motor operates under heavy loads.
In addition to size and efficiency, another important factor in many motor applications is the smoothness of the motor torque output curve. Smooth torque output is interrupted by cogging or detent torque, and by EMF ripple. Cogging torque is caused by periodic alignment of attractive magnetic elements. EMF ripple results from the fact that all of the stator poles energized in a given commutation state are not "in phase" with each other. The greater the phase difference, the greater is the EMF ripple. Also, the frequency of these interrupting torques is important, with higher frequencies being less objectionable.
In addition to the above, a motor must be designed with sufficient minimum starting torque for its intended application.
Therefore, while there is no "ideal" brushless DC motor, many users of motors would like a motor of reduced size with increased efficiency that keeps torque ripple within acceptable limits and has acceptable starting torque. Of course, such a motor would have to be capable of being economically manufactured and would ideally be compatible with existing and proposed available power sources.
Yet another consideration is selection of the type of circuitry to be used to supply electrical power to the motor. This decision is based upon a trade off of circuit complexity against the desirable characteristics of smooth running torque, self-starting action, and high starting torque. For applications such as those intended for the inventive motor, 3-phase bipolar electrical power circuitry has been found to be an optimal compromise and is widely used in the field.
Yet another consideration is the fact that there is a demand to make motor poles as small as possible in order to minimize the overall height of the motor, but that this effort is often in conflict with other overall objectives such as smooth operating torque.
All prior art brushless DC motors within the inventor's knowledge are either too large in at least the height dimension or too inefficient to be well suited for such applications as fixed disk drive motors in portable battery operated computers, or else they otherwise suffer undesirable properties such as excessive asymmetrical radial forces or excessive torque ripple. No prior art DC motor to the inventor's knowledge has successfully combined both the small size and the efficiency of the present inventive motor while maintaining acceptable torque characteristics and preventing the production of excessive asymmetrical radial forces.